The demand for wireless communication services such as Cellular Mobile Telephone (CMT), Digital Cellular Network (DCN), Personal Communication Services (PCS) and the like typically requires the operators of such systems to serve an ever increasing number of users in a given service area. As a result, a type of base station equipment known as a multichannel broadband transceiver system (BTS) has been developed which is intended to service a relatively large number of active mobile stations in each cell. Such a broadband transceiver system can typically service, for example, ninety-six simultaneously active mobile stations in a single four-foot tall rack of electronic equipment. This base station equipment typically costs less than $2000 to $4000 per channel to deploy, and so the cost per channel serviced is relationally low.
When coupled with efficient frequency reuse schemes, such as that described in a co-pending U.S. patent application Ser. No. 08/331,455 entitled "A Method For Obtaining Times One Frequency Reuse in Communication Systems" filed by John R. Doner on Oct. 31, 1994 and assigned to AirNet Communications Corporation, who is the assignee of the present application, maximum efficiency in densely populated urban environments is obtained. According to that arrangement the cells are each split into six radial sectors and frequencies are assigned to the sectors in such a manner as to provide the ability to reuse each available frequency in every third cell. Although this frequency reuse scheme is highly efficient, it requires at least two complete sets of multichannel transceiver equipment such as in the form of a broadband transceiver system (BTS) to be located in each cell.
An important practical consideration however, is that when a wireless system first comes on line, demand for use in most of the cells is relatively low. It is therefore typically not possible to justify the cost of deploying complex multichannel broadband transceiver system equipment based only upon the initial number of subscribers. Because only a few cells at high expected traffic demand locations (such as in a downtown urban location or a freeway intersection) will justify the expense of building out a network of high capacity broadband transceiver systems the service provider is faced with a dilemma. He can buildout the system with less expensive narrowband equipment initially to provide some level of coverage, and then upgrade to the more efficient equipment as the number of subscribers rapidly increases in the service area. However, the initial investment in narrowband equipment is then lost. Alternatively, a larger up front investment can be made to deploy the high capacity equipment at the beginning, so that once demand increases the users of the system can be accommodated without receiving busy signals and the like. But this has the disadvantage of carrying the money cost of a larger up front investment.
Some have proposed various techniques for extending the service area of a given cell. For example, U.S. Pat. No. 4,727,390 issued to Kawano et al. and assigned to Mitsubishi Denki Kabushiki Kaisha discloses a mobile telephone system in which a number of repeater stations are installed at the boundary points of hexagonally shaped cells. The repeaters define a small minor array which is superimposed on a major array of conventional base stations installed at the center of the cells. With this arrangement, any signals received in so-called minor service areas by the repeaters are relayed to the nearest base station.
Another technique was disclosed in U.S. Pat. No. 5,152,002 issued to Leslie et al., and assigned to Orion Industries, Inc., wherein the coverage of a cell is extended by including a number of so-called "boosters" arranged in a serial chain. As a mobile station moves along an elongated area of coverage, it is automatically picked up by an approaching booster and dropped by a receding booster. These boosters, or translators, use highly directive antennas to communicate with one another and thus ultimately via the serial chain with the controlling central site. The boosters may either be used in a mode where the boosted signal is transmitted at the same frequency as it is received or in a mode where the incoming signal is retransmitted at a different translated frequency.
Unfortunately, each of these techniques have their difficulties. With the first method, which uses an array of repeaters co-located with the primary cell sites, the implementation of diversity receivers becomes a problem. Specifically, certain types of cellular communication systems, particularly those that use digital forms of modulation, are susceptible to multi-path fading and other distortion. It is imperative in such systems to deploy spatial diversity antennas at each cell site in order to minimize multipath distortion. Unfortunately, the repeater array scheme makes implementation of diversity antennas extremely difficult, since each repeater simply forwards its received signal to the base station. Diversity information as represented by the phase of the signal received at the repeater location is thus lost.
The second scheme works fine in a situation where the boosters are intended to be laid in a straight line such as along a highway, a tunnel, a narrow depression in the terrain such as a ravine or adjacent a riverbed. However, there is no teaching of how to efficiently deploy the boosters in a two-dimensional grid, or to share the available translated frequencies as must be done if the advantages of cell site extension are to be obtained throughout an entire service region, such as a large city. This second scheme also suffers from the same problem as the first in that there is a loss of diversity information at the booster locations.